Discussion
Understanding fungal response to climate warming is important for
revealing their roles in nutrient cycling processes and evaluating the
ecological consequences of climate warming on high-latitude tundra. In
this study, we hypothesized that climate warming would affect fungal
community composition and potential interactions. Our hypotheses were
partially upheld: fungal functional gene composition and potential
interactions among genes/species were significantly altered, whereas
fungal community composition was unchanged (Table 1 & Fig. 2).
Different responses of functional gene composition and community
composition were similar to the findings of a previous warming study in
a tall-grass prairie ecosystem (Cheng et al., 2017), suggesting that
GeoChip is more sensitive for detecting subtle changes in functional
genes. Due to horizontal gene
transfer among microbial species, species identities detected in GeoChip
do not reliably represent phylogenetic species in environmental samples.
However, if both sequencing and GeoChip data point to the same species,
the chance of disconnection between sequencing and GeoChip is slim. For
example, we detected both functional genes and OTUs belonging toHypocrea in GeoChip data and amplicon sequencing data. Similar to
the overall changes of fungal composition change, we found significant
increases in gene abundances from Hypocrea (e.g., the genes
encoding endochitinase, exopolygalacturonase, and xylanase) (P< 0.05), whereas no changes of OTUs from this genus.
Nonetheless, the lack of
detectable changes in fungal community composition should be interpreted
with caution because 28S rRNA gene has a relatively low taxonomic
resolution (Brown, Rigdon-Huss, & Jumpponen, 2014), which might fail to
detect subtle changes in fine taxonomic levels (Hultman et al., 2015).
Ascomycota was the dominant fungal phylum in our study and other
studies conducted in sedge-occupied tussock tundra (Christiansen et al.,
2016; Deslippe, Hartmann, Simard, & Mohn, 2012; Timling, Walker,
Nusbaum, Lennon, & Taylor, 2014).
However, this phylum was less
dominant in shrub-occupied tundra
(Wallenstein, McMahon, & Schimel, 2007), indicating that vegetation
pattern is probably closely related to fungal community composition.
Climate warming has caused vegetation shifts in the tundra, with
diminishing sedges and bryophytes but expanding shrubs and trees
(Fraser, Lantz, Olthof, Kokelj, & Sims, 2014; Sturm et al., 2001).
Considering the potential relationship between fungi and plants, it is
likely that dominant fungi in high-latitude tundra will shift
accompanying the shrub expansion.
The increase in fungal C
degradation capacities, based on higher relative abundances of
functional genes encoding invertase, xylose reductase, and vanillin
dehydrogenase (Fig. 1), might imply that fungi-mediated C degradation
was accelerated under warming. Those genes are associated with the
degradation of complex plant-derived saccharides (Culleton, Mckie, & de
Vries, 2013), probably resulting from higher plant productivity (Table
S1). Moreover, higher vanillin dehydrogenase gene abundance might result
from the “priming effect,” i.e., labile C input can stimulate
recalcitrant C degradation (de Graaff, Classen, Castro, & Schadt, 2010;
Mau, Dijkstra, Schwartz, Koch, & Hungate, 2018). Warming-induced
increases of fungal C degradation capacities were also found in other
ecosystems including grasslands (Cheng et al., 2017), subtropical
freshwater wetlands (Wang et al., 2012), deserts (Weber et al., 2011),
showing a consistent response pattern of fungal communities across
ecosystems. For example, in the biological soil crusts of a desert, soil
warming increased the abundance of fungal functional gene cbhI ,
which encodes cellobiohydrolase for cellulose degradation (Weber et al.,
2011).
Higher average path distance but lower average clustering coefficient of
the warmed fMEN (Table 3) suggested that the complexity of potential
interactions decreased under warming. As the percentage of negative
links could represent competition within community members (Fuhrman,
2009), a lower percentage of negative links in warmed fMEN concurred
with higher soil nutrient availability that alleviates resource
competition (Banerjee et al., 2016). Higher nutrient availability
appears to stimulate C degradation, as almost all key genes, the most
important members in maintaining network structure and strongly
influence community stability and functions (Banerjee et al., 2018; Shi
et al., 2016), in warmed fMEN were associated with C degradation (Table
S4). However, it is important to
bear in mind a caveat that most networks cannot distinguish true
ecological interactions based on positive or negative correlations
between nodes since it remains largely intractable to analyze in
situ microbial interaction experimentally in most communities of
natural environments (Faust & Raes, 2012). When interpreting networks
in ecological terms, topological properties (e.g., average node
connectivity, average path length, clustering coefficient, and
modularity), which reflect whole-network changes, could be more
reliable.
Legacy effects of winter warming included increases in thaw depth, soil
moisture, and GPP in the growing season (Table S1). The increase of thaw
depth, a sign of permafrost degradation, was also observed in previous
experimental warming studies using snow fences (Hinkel & Hurd, 2006;
Nowinski, Taneva, Trumbore, & Welker, 2010). Since we excluded snow
addition effects on soil hydrological conditions by removing snow before
melting, the most likely cause of soil moisture increase was ice-wedge
melting in permafrost (Liljedahl et al., 2016). Soil temperature, soil
moisture, thaw depth, and GPP imposed the strongest influence on fungal
functional gene composition (Fig. 3), suggesting that they were very
important in affecting fungal functional capacities. Oxygen availability
could be affected by soil hydrology and temperature, which is shown to
change species diversity, ecological functions, and survival of most
aerobic fungi (Wang et al., 2012; Zak & Kling, 2006).
Permafrost thawing increases the potential of old C degradation, leading
to stronger heterotrophic respiration and CO2 emission
(Nowinski et al., 2010; Schuur et al., 2009). Consistently, we found a
significant warming-induced increase in R eco(Table S1 & Fig. 4), primarily resulting from the increase in winterR eco that mainly represents heterotrophic
respiration (by 103.2%, P< 0.05, Table S1). Our
winter R eco data were derived from a
site-specific model that assumes winter R ecoincreases with soil temperature exponentially, which is commonly used
when simulating the temperature dependence of heterotrophic soil
respiration. The model fitted in situ measured data well
(R 2 = 0.70, P < 0.001), which
was consistent with observations elsewhere (Tuomi, Vanhala, Karhu,
Fritze, & Liski, 2008).
Increased R eco shared positive regressions with
fungal functional genes for C degradation (Fig. 4), suggesting that
fungal functional capacities in C degradation might be very important in
mediating R eco, and thus the C stability of
tundra ecosystems.
According
to the central dogma, DNA has to be transcribed into RNA and translated
into protein before displaying enzymatic activity, which is the missing
link in our study since enzyme activities were not measured. However, a
large number of papers have justified the strong, positive relationship
between enzyme activities and abundances of their encoding genes
(Blackwood, Waldrop, Zak, & Sinsabaugh, 2007; Fan, Li, Wakelin, Yu, &
Liang, 2012; Trivedi et al., 2016).
Notably, the positive correlations
between R eco and fungal functional genes should
be interpreted with caution since GPP was also increased by warming
(Table S1). Therefore, the increase of R eco could
be explained in terms of the increased photosynthesis and corresponding
increased respiration of new photosynthate.
Additionally, higher bacterial C
degradation capacities under warming conditions at this site have been
previously documented (Xue et al., 2016), which could also account for
the R eco increase. Therefore, future studies that
distinguish plant respiration and heterotrophic soil respiration are
needed.